1. Field of the Invention
The present invention relates to a medium supplying apparatus that supplies a medium and an image forming apparatus including the medium supplying apparatus.
2. Description of the Related Art
In an image forming apparatus such as an electrophotographic or ink-jet type copying machine, printer or the like, a paper housed inside the image forming apparatus is transferred, an image is formed on the paper by an image forming unit using toner, ink or the like, and then the paper is ejected outside the image forming apparatus.
In such an image forming apparatus, a rolled medium (hereinafter, referred to as a “rolled paper”), which is a rolled continuous sheet, is often used when printing the image on a large size paper such as A0 or A1. By using such a rolled paper, it is easy to handle the paper. Additionally, the size of the image forming apparatus with respect to the size of the paper can be made smaller.
When using the rolled paper, a transfer unit that holds the rolled paper with two pairs of transfer rollers is provided and the rolled paper is transferred from a housing of the rolled paper to the image forming unit by the transfer rollers of the transfer unit.
At this time, if the rolled paper is not attached to be in parallel with respect to the transfer rollers, a direction of a force transmitted by the transfer roller to the rolled paper and a moving direction of the rolled paper become different. At that time, the rolled paper may be transferred while being inclined with respect to the moving direction.
Such an inclination may also occur by looseness of the rolled paper which is caused when the transfer unit terminates the transferring of the rolled paper and the rolled paper is rotated by inertia, in addition to the above case which is caused when the rolled paper is attached. When the inclination is severe, wrinkling or twisting may occur on the rolled paper when the rolled paper is transferred such as in a forward direction or a reverse direction.
Thus, in order to suppress the generation of the wrinkling of the rolled paper caused by the inclination, a method in which a friction member is pushed toward an edge or a surface of the rolled paper, or a load is applied to an axial core of the rolled paper, which is being transferred in the forward direction, by a torque limiter or the like, is known.
By applying a back tension toward an opposite direction from the transfer direction of the rolled paper by applying the load using the friction member, the torque limiter or the like, the inclination of the rolled paper is corrected by a transferring force by the transfer rollers and the back tension to suppress the generation of the wrinkling.
The mechanism of correcting the inclination of the rolled paper by applying the back tension is explained with reference to FIG. 18. In FIG. 18, the lower side of a rolled paper R is tensioned between transfer rollers A and the rolled paper R, while the upper side of the rolled paper R is loosened. At this time, when the back tension is applied in a direction opposite to a transferring direction “T” of the rolled paper R, a transferring load “TL”, which becomes lesser at the loosened side and greater at the tensioned side, is applied to the rolled paper R.
At this time, the transferring force by the transfer rollers A toward the rolled paper R becomes constant in the axial direction. Thus, an effective transferring force which is actually applied to the rolled paper R becomes a difference between the transferring force and the back tension and is expressed as an actual transferring force “ATF”.
By applying the back tension, the actual transferring force “ATF” to transfer the rolled paper R can be greater at the loosened side and lesser at the tensioned side of the rolled paper R. Thus, the inclination can be corrected in a direction shown by an arrow “C”.
However, as an appropriate strength for the back tension is different based on the size of the rolled paper, the remaining amount of the rolled paper or the like, the inclination of the rolled paper cannot be sufficiently corrected by applying a constant back tension to a rotation shaft of the rolled paper.
Thus, Patent Document 1, for example, discloses a technique in which a torque limiter is provided at a spool which rotatably supports the rolled paper to apply a rotation load when the rolled paper is transferred in a forward direction and providing a transferring force variable means, which is an electromagnetic clutch, that varies the transferring force of the rolled paper. In this technique, voltage to be applied to the electromagnetic clutch is continuously determined based on the detected widths of the rolled paper to control the back tension to enable a correction of an inclination in detail.
In Patent Document 2, a method in which a powder clutch is provided that engages a flange of a rolled paper to apply a back tension, an electric power supplied to the powder clutch is controlled based on an output of a remaining amount detection unit for the rolled paper, and the strength of the back tension applied to the rolled paper is varied, is disclosed.
However, by the techniques disclosed in Patent Document 1 or Patent Document 2, the electromagnetic clutch such as the powder clutch or the like is necessary so that the cost is increased and the structure and control for applying the back tension becomes complicated. Further, there may be a case when the inclination of the rolled paper caused by the looseness generated when the rolled paper is rotated by inertia after the transferring of the rolled paper by the transfer unit is terminated.